Metastasis of tumor cells from the primary site of a malignancy accounts for the majority of fatalities in cancer patients. The process of metastasis is complex and involves several steps including invasion and degradation of the extracellular matrix, intravasation, transit through the vasculature, extravasation, and establishment of new tumor foci distant from the primary tumor. These steps probably involve the actions of several proteolytic enzymes which are regulated at several levels temporally during the metastatic process. UPA is among pivotal enzymes involved in the metastic process through its ability to activate plasminogen to plasmin. The activity of uPA in tumor cells is probably regulated by the binding of uPA with the inhibitor PAI-1. Preliminary data from a human osteosarcoma model comprised of cell lines showing varying abilities to form tumors and metastasize in athymic nude mice has shown that the level of membrane bound UPA rather than the level of secreted uPA correlates with the invasive and metastatic behavior of the tumor cells. By Southern blot technique using HEL299, a diploid human embryonic cell line, as a control for gene copy number, we have found that the genes encoding PAI-1, and uPAR are single copy in HOS, AD110, and KRIB cells By Northern blot hybridization, AD110 cells express approximately 2-3 fold more steady state PAI-1 mRNA than either HOS or KRIB cells. AD110 and KRIB express equivalent levels of uPAR mRNA (approximately 5-10 fold greater than HOS cells). Further studies are underway to evaluate the transcriptional regulation of the gene encoding PA1-1 in these cells. The role of PAI-1 in regulating the binding of uPA to uPAR and the effect of PAI-1 on the invasive and metastatic properities of AD110 will be evaluated.